1. Field of the Invention
This invention relates to fluorescent lamps and is directed more particularly to an amalgam assembly including an improved amalgam retainer for use within an exhaust tubulation of a fluorescent lamp.
2. Description of the Prior Art
The light output of fluorescent lamps is critically dependent upon mercury vapor pressure (vapor density) within the lamp envelope. The mercury vapor pressure, in turn, is controlled by the temperature of excess liquid mercury which condenses in the coldest part of the lamp envelope, the so-called xe2x80x9ccold spotxe2x80x9d. Fluorescent lamps typically include at least one tubulation that has an opening into the interior of the lamp envelope and which, in construction of the lamp, is used as an exhaust and fill tubulation. At completion of manufacture, the exhaust tubulation is hermetically tipped off and the tipped end typically becomes the lamp xe2x80x9ccold spotxe2x80x9d.
The amalgam is commonly located in the exhaust tubulation cold spot. Such amalgams reduce the mercury vapor pressure relative to that of pure mercury at any given temperature and thereby permit optimum light output at elevated temperatures. Such amalgams also provide a broadened peak in the light output versus temperature curve, so that near optimum light output is obtained over an extended range of ambient temperatures.
When lamps are operated at temperatures lower or higher than the optimum ambient temperature, light output decreases by as much as 30% or more relative to peak value. This is a common occurrence when lamps are operated in enclosed or semi-enclosed fixtures. In addition to reduced light output, the color of the light varies as a result of the varying contribution of blue spectral emission from the mercury vapor in the discharge.
The problem of mercury vapor pressure control under varying temperature conditions is solved, at least in part, through the use of various alloys capable of absorbing mercury from its gaseous phase. Alloys of low temperature melting metals are often placed within fluorescent lamps to amalgamate with the excess mercury, and to regulate the mercury vapor pressure within the lamp. Alloys known to be particularly useful in forming amalgams with mercury include a lead-bismuth-tin alloy, a bismuth-indium alloy, a bismuth and tin alloy, and a zinc, indium and tin alloy. Other useful amalgams may be formed with pure indium, pure lead, and pure zinc.
The lamp typically is provided with an excess amount of mercury amalgam, that is, more amalgam than is needed to supply the mercury vaporized when the lamp reaches a stabilized operating condition. As the lamp ages, some of the excess amalgam is required to replace the mercury chemically bound elsewhere in the lamp during the life of the lamp.
When an amalgam fluorescent lamp is turned off, the amalgam cools and the mercury vapor within the lamp is gradually absorbed into the amalgam. When the lamp is turned on, the lumen output is significantly reduced until the amalgam is warmed up to a point at which the amalgam emits sufficient mercury vapor to permit efficient lamp operation.
In some types of lamps, particularly electrodeless fluorescent lamps, it is important that the amalgam be prevented from settling within the arc environment in the lamp envelope where the amalgam can cause deleterious changes in the lumen output and the lumen-temperature performance of the lamp.
In base-up lamps, there has been a particular problem in that in use the sealed end of the tubulation is pointed upwardly and the end of the tubulation that opens into the lamp envelope is disposed downwardly of the amalgam, and the amalgam has tended to drop by gravity downwardly into the lamp envelope, where a much higher temperature is present, causing a sudden rise in mercury vapor pressure and an increase in lamp voltage, resulting in the occurrence of black spots on the glass envelope. If the lamp voltage exceeds the maximum sustaining voltage of the ballast provided in the lamp, the lamp extinguishes. There is thus required a means for retaining liquid amalgam in the tubulation, but permitting mercury vapor to exit the tubulation and flow into the lamp envelope.
Accordingly, there is a need for an amalgam assembly including an amalgam retainer for limiting the amalgam to the tubulation sealed end region.
An object of the invention is, therefore, to provide an amalgam assembly featuring an improved retainer for disposition in an exhaust tubulation of a fluorescent lamp to prevent migration of liquid amalgam into the lamp envelope.
A further object of the invention is to provide an electrodeless fluorescent lamp having therein an amalgam assembly featuring an improved amalgam retainer.
With the above and other objects in view, as will hereinafter appear, a feature of the present invention is the provision of an amalgam assembly for a fluorescent lamp. The assembly comprises a glass exhaust tubulation extending toward a base portion of the lamp, the tubulation being closed at an end adjacent the base portion. A metal cup is disposed in the tubulation and is retained by a pinched portion of the tubulation. The cup defines an annular outer wall having a free edge extending toward the tubulation closed end, a tubular central core portion extending toward the tubulation closed end, and an annular trough formed by the core portion and the outer wall. A mercury amalgam ball is disposed between the metal cup and the tubulation closed end, a diameter of the ball exceeding an inner diameter of the core portion, and a coating of a metal wetting agent is disposed on interior surfaces of the trough. When the amalgam body liquidizes, the liquid amalgam adheres to the cup trough surfaces and mercury vapor is flowable through the cup core portion.
In accordance with a further feature of the invention, there is provided an electrodeless fluorescent lamp comprising a light-transmissive envelope containing an ionizable, gaseous fill for sustaining an arc discharge when subjected to a radio frequency magnetic field and for emitting ultraviolet radiation as a result thereof. The envelope is provided with an interior phosphor coating for emitting visible radiation when excited by the ultraviolet radiation, and with a re-entrant cavity formed therein. An excitation coil is contained within the re-entrant cavity for providing the radio frequency magnetic field when excited by a radio frequency power supply. An exhaust tubulation extends through the re-entrant cavity and into the envelope for evacuating and filling the lamp in manufacture, the exhaust tubulation extending toward a base portion of the lamp and having a closed end proximate the lamp base portion. A dimple configuration is formed in the exhaust tubulation at a predetermined distance from the tubulation closed end. A metal cup retains an amalgam in a location in the exhaust tubulation between the metal cup and the closed end of the exhaust tubulation, the metal cup defining an annular wall having a free edge extending toward the exhaust tubulation closed end, a tubular central core portion extending toward the exhaust tubulation closed end, and an annular trough formed by the core portion and the outer wall. A coating of a metal wetting agent is disposed on interior surfaces of the trough. When the amalgam body liquidizes, the liquid amalgam adheres to the cup trough surfaces and mercury vapor is flowable through the cup core portion.